The present invention relates to an ignition timing control apparatus for an internal combustion engine, and more particularly, it relates to an ignition timing control apparatus for sensing an abnormality in a knock sensor in the form of a vibrational acceleration sensor.
Generally, it is desirable to set the ignition timing of an internal combustion engine in such a manner as to maximize the operating efficiency of the engine. Thus, it is general practice to set the ignition timing as close as possible to a maximum advance angle or position within an allowable range in which there takes place no knocking in the engine. In this connection, however, most of conventional ignition timing control apparatuses are of the mechanical type and generally have non-uniform and unstable ignition advance characteristics due mainly to relatively significant variations in manufacture and gradual changes in quality over time. Accordingly, in order to prevent possible knocking in the engine, there is a tendency that ignition timing is in effect set at a point considerably retarded from the most desirable advance position.
In this case, however, the operating efficiency of the engine is reduced. In addition, even in the event that ignition timing is set to a point at which there will be no knocking in a certain operating condition of the engine, by use of an ignition timing control apparatus, which is precisely manufactured with uniform quality and is highly resistant to time-dependent changes, there still exists a probability of engine knocking because whether knocking will take place or not depends on many factors such as the temperature and humidity of intake air, the air/fuel ration of a mixture, etc.
However, such a probability can be eliminated by sensing knocking in an engine and properly controlling the ignition timing in an ignition retarding direction if it is sensed. According to this ignition control, it becomes possible to properly control the ignition timing so as to prevent knocking even if there are variations in the ignition advance characteristics of ignition control timing apparatuses due to mechanical variations among them or due to changes in the engine operating condition. To this end, a reference ignition timing, at which ignition usually takes place when there is no knocking, is set to a point close to the maximum advance position, and if knocking takes place, ignition timing is appropriately retarded to provide optimum ignition timing for preventing the knocking.
Even with this type of ignition timing control apparatus, there arises a problem when a knock sensing system becomes abnormal or faulty. Specifically, if there develops an abnormality in the knock sensing system, no information about knocking could be obtained, so it becomes impossible to perform accurate ignition retarding control for knocking suppression if the engine is knocking. As a result, the engine is subject to severe vibrations due to knocking, which might cause serious damage to the engine.
To cope with such a problem, if there is an abnormality in the knock sensing system, it is desirable to control the ignition timing in an ignition retarding sense so as to prevent an occurrence of knocking or alert the operator of the engine of such an abnormality.